Photoelectron multiplier

ABSTRACT

The invention provides a photodetector responsive to modulated laser light. A multiplier phototube is used having a photocathode and a dynode arranged on the same plane. An electric field is used to accelerate the electrons from the photocathode and a high frequency standing wave is provided adjacent to the dynode. The trajectories of the electrons are phase compressed and the electron stream is highly localized.

PIP-3105 Inventor Masao Miya Tokyo, Japan Mar. 6, 1968 Apr. 6, 1971 Nippon Electric Company, Limited Tokyo-to, Japan Oct. 13, 1967 Japan Appl. No. Filed Patented Assignee Priority PHOTOELECTRON MULTlPLlER 5 Claims, 2 Drawing Figs.

US. Cl 250/199,

313/103, 315/525, 315/393 Int. Cl 1104b 9/00 Field of Search 250/ 199;

313/103; 315/393; 315/525, (lnquired) [561 References Cited UNITED STATES PATENTS 3,388,282 6/1968 Hankin et al. 313/103 3,431,420 3/1969 Fisher et al. 315/393 Primary Examiner-Robert L. Richardson Assistant Examiner-Albert J. Mayer Attorney-Hopgood and Calimafde ABSTRACT: The invention provides a photodetector responsive to modulated laser light. A multiplier phototube is used having a photocathode and a dynode arranged on the same plane. An electric field is used to accelerate the electrons from the photocathode and a high frequency standing wave is provided adjacent to the dynode. The trajectories of the electrons are phase compressed and the electron stream is highly localized.

Patented April 6, 1971 'fQf? J JWHH:

FIG.|

ATTORNEYS INVIJN'IUR. M4540 Ml YA response of the multiplier.

Various methods have been proposed to obviate this difficulty. For example, a photomultiplier utilizing the motion of electrons in a crossed electric and magnetic fields is described in a paper disclosed in the IEEE Journal of Quantum Electronics, Apr. I965, pp. 4959. In the disclosed device, the influence of the random initial emission velocities is eliminated by the utilization of the fact that all the electrons simultaneously emitted from the same origin return to the plane of origin simultaneously, regardless of the emission velocity. The trajectories are trochoidal for electrons of finite emission velocity, while cycloidal for electrons with zero velocity.

As a practical matter, however, the arrangement of the dynodes disposed within a plane cannot cause sufficient electron emission to induce the emission at the next adjacent dynode stage. To give the electrons sufficient energy to cause the succeeding secondary electron emission, the dynodes should be disposed in a staircase configuration. In such a configuration, however, the complete elimination of the influence of the random emission velocity is impossible. Moreover, the residual effect accumulates as the number of dynodes increases. Also, since the radius of the circular trajectory of an electron varies in proportion to its initial emission velocity, the electrons, even if emitted from one point of a dynode stage, arrive at different points at the next stage, causing spatial dispersion. Furthermore, this dispersion also considerably accumulates as the number of the stages increases. Consequently, it is difficult with the disclosed device to design the electrode for finally collecting the electron beam having been subjected to secondary electron emission multiplication.

Another example is a dynamic crossed-field electron multiplier proposed in a paper disclosed in the Proceedings of the IEEE, Jan. 1963, pp. l53l62. The device utilizes the behavior of an electron moving in the mutually crossed microwave electric and static magnetic fields. This device suggests a solution to the above-mentioned problem of the electron transit time dispersion caused by the random emission velocity, and provides a means for causing electron bunching on the time axis. With this device, the energy for emitting the secondary electrons is transferred to each of the emitted electrons from the microwave electric field, the mutual phases of the electrons emitted from the preceding state are relatively compressed owing to the bunching action, causing more inten- I sified collision of electrons against the next-stage dynode disposed on the same plane. Therefore, this device is operative at the microwave frequency with the driving electric field of the appropriate microwave frequency, because the electron transit time is precisely controlled. However, this device does not include any means for suppressing the spacial dispersion of electrons which accumulates as the number of the stages increases, as is the case with the above-mentioned photomultiplier of the static electric and magnetic fields type. Also, unavoidable inclusion of the photocathode and dynodes within the microwave cavity resonator adversely affects the characteristics of the device as a microwave circuit and makes its fabrication difficult. Occasionally, the oscillation is caused by the multitransit of the stray electrons.

The object of the present invention is, therefore, to provide an improved photomultiplier free from any defects of the above-mentioned conventional photomultipliers of the crossed electric and magnetic fields type.

The present invention is based on the above-mentioned principle of the photomultiplier of crossed electric (high frequency) and magnetic fields type, and provides an improvement in that the electric field is produced by the standing wave of microwave frequency. Owing to the existence of the microwave standing-wave field, the trajectories of the electrons are successfully phase compressed, with the result that the intensification of the successive secondary electron emissions are facilitated.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawing, wherein:

FIG. 1 is a diagram showing the relations between the trajectories of the electrons and the high frequency electric field intensity, used for explaining the principle of the invention; and

FIG. 2 shows a longitudinal sectional view of the structure of an embodiment of the invention.

Referring to FIGS. 1 and 2 simultaneously, a microwave having an angular frequency a) and an electric field component 1 in the y-axis direction propagates in both directions along the axis of y=0 or along the z-axis. Therefore, a standing-wave electric field represented by an expression:

E=2E sin wrcos Bz (l) is formed along the z-axis as a whole, where E, and B stand for the peak value of the amplitude of the microwave electric field and its phase propagation constant, respectively. Static magnetic field Bis applied in the direction perpendicular to the plane of the sheet of drawing or, in other words, in the x-axis direction. Under the situation, photoelectrons 4 are emitted, as a result of the introduction of the modulated laser light beam 3 through a transparent window 8, from a photocathode 2 disposed on the y=d plane parallel to the z-axis and with a velocity v accelerated by a direct-current voltage V applied across planes y=0 and y='d. Since the electron beam is subjected to the velocity-modulation caused by the standing-wave electric field of equation (1) existent along the z-axis, the velocity v of an electron crossing the point z, at time t is represented by an equation:

1 a (8) B (s) where m and e stand for the mass and electric charge of an electron, respectively. The lapse of time T until the return of the electron to another point on the z-axis is represented by an equation:

This means that the lapse of time T does not depend on the emission point z on the dynode 6 or the emission velocity v,. More particularly, all the electrons having simultaneously departed from the z-axis simultaneously return to the z-axis, regardless of the random emission velocities. In as much as the standing-wave electric field shown by equation (I) is existent, however, the radius of the circular trajectory becomes large in proportion to the increase in velocity, which is caused by the velocity modulation due to the standing wave electric field. As is obvious from the equation (3) and FIG. 1, it is possible, among the electrons being simultaneously emitted from the mutually different points on the z-axis (dynode 6) and again return to other points on the z-axis, that an electron emitted from a point behind another electrons simultaneous emission point (with respect to the z-axis) returns to the z-axis at a after being accelerated by direct-current voltage V applied point ahead of said another electrons returning point, as a between the dynode 6 and the lower wall of the waveguide 9. result of the spatial diflerences in accelerating field intensity. On the other hand, a microwave having the transverse electric Therefore, as shown in FIG. 1, the electrons can be bunched field is supplied to the wavequide through window 11 disposed along the z-axis by subjecting the electrons repetitively to the at the end of waveguide 9 for forming a part of the vacuum enacceleration phase of the standing-wave electric field. velope. The wavelength and phase of the microwave are More particularly, since the electrons returning to the z-axis selected so that a standing wave may be formed under the inhave been twice subjected to velocity-modulation by the fluence 0f reflection caused by plate 12 disposed at the end. standing-wave electric field on the z-axis, the resultant incre- The electrons pass through an elongated central opening 12' ment AE of the kinetic energy of an electron is given by: 10 formed in plate 12 into gap 10, and are subjected to velocity modulation during their traveling in the gap 10 from photocathode 6 to waveguide 9 and then caused to return to the gap 10 following a circular trajectories owing to the in- 2mvo m fluence of the magnetic field B. The velocity-modulated elec- +cos YE X 81h E)] (5) 15 trons finally collide against the dynode 6 to emit the secondary electrons 7. As these secondary electrons repeatedly move in Therefore, the transfer of energy from the electric field to an a manner identical to lhal 0f the r g Photoeleclmhs, the electron takes place twice, increasing the electrons kinetic number of the electrons increases in a Well-known mannerenergy as a whole. Thus, the electrons which have acquired After repeating the same p the electrons are collected the additional kinetic energy collide against the dynode 6 hyaconecml'13-Thusithfiphotoelecme'convenedandmul spaced b l h baxis 111 dynode emits the Sewntlplied or amplified electric signal rs derived from an output dary electrons 7, the number of which is multiplied according lemma] 14 connect? to the eleeh'ode to the secondary emission ratio determined by the material of As has been expla'med P 3 p i p j havlhg a dynode The emmgd secondary electrons are accelerated by simple construction is provided according to the invention, the direct-current voltage V in the +y-direction to reach the i shhswhhhhy obvletes the adverse effects f the random z-axis again. On this occasion, it is possible to bring the elecehhssleh veloeliy of the i h More Pamehlahy the tron trajectory into coincidence with the acceleration phase of eh has made h peeslblefe he the electrons the high-frequency electric field, by suitably choosing the ehhhed at rahdem h ehheeloh pelhts and electron transit time betwefin and y=0 The aboveqnem velocities. Thus, the present invention has provided a (med process is repeated to the end of the cycloidal or photodetector of high sensitivity responsive to a wide band trochoida] trajectory modulating signal extending to the microwave frequency re- As will be understood from the fore oin ,the ma etic field gioh' B, direcwunem voltage V0 and g d began the While the foregoing description sets forth the principles of the invention in connection with specific apparatus, it is to be t {23:5 222x385: al. 553 1,; glif aff satisfy the fol understood that this description rs made only by way of exam- (1) f ple and not as a limitation of the scope of the invention as set forth in the Objects thereof.

lclaim:

The reason: The transit time T of the electron on the l. A multiplier phototube comprising:

an envelope, a photocathode for emitting electrons, an semicircular trajectory must be equal to the period of oscillation anode for collecting said electrons, at least one dynode to provide secondary emission, said photocathode, dynode 2 N and anode being contained within said envelope and T=' 5 defining a drift space through which said electrons may move along chained-semicircular trajectories, said dynode being disposed within substantially the same N being an integer of the high-frequency electric field. plane as Said photocathode, a waveguide section disposed (2) V 1 0 MB 2 substantially parallel to said dynode and having one of its "2' 7 ends shorted for providing a high frequency standing electric wave within said drift space along said dynode to The reason The rad'us of the trajectory accelerate said electrons, said waveguide section having a R0: v0 gap for allowing said standing electric wave to interact (2 B with said electrons, said gap being smaller than the m distance of the furthest end of each of said seirnicircular trajectories from said dynode as measured in the must be equal ohe'fohhh of the ghde wavelength h direction perpendicular to the axis of said waveguide sec- (3 N 6 1/2 tion, and means for applying a static magnetic field within =1? said drift space to deflect the accelerated electrons,

whereby said standing wave means further causes said accelerated deflected electrons to bunch. 2. The phototube of claim 1, in which said waveguide section comprises, a ridge waveguide for forming said gap.

3. The phototube of claim 1 including, DC-biasing means coupled to said dynode.

4. The phototube of claim 3 in which said DC-biasing means provides a voltage V and the electrons follow a semicircular The reason: the transit time 10 of the electron covering the distance 4, which is equal to e 1/ 2 provided that the influence of the magnetic field is neglected, must be equal to the oscillation period 1 of the high-frequency trajectory, the magnetic field B, direct-current voltage V and eleclfic fieldspace 1 between the photocathode and z-axis approximately Now, an embodiment of the present invention is explained satisfying the following conditions: with particular reference to FIG. 2.

In FIG. 2, a transmitted laser beam 3 is introduced to a photocathode 2 through a transparent window 8 attached to the envelope of the device. Photoelectrons 4 emitted from the B'=-" photocathode 2 pass through a gap 10 of a ridge waveguide 9 m the radius of the trajectory being equal to one-fourth of the guide wavelength A g;

the transit time 1-0 of the electron covering the distance 1 equal to 4d electric field.

5. A system for demodulating a modulated laser light beam means directing the light beam to said photocathode; and anode for collecting said electrons; at least one dynode to provide secondary emission; said photocathode, dynode and anode being contained within said envelope and defining a drift space through which said electrons may move along chained-semicircular trajectories; said dynode being disposed within substantially the same plane as said photocathode; a

waveguide section disposed substantially parallel to said dynode and having one of its ends shorted for providing a high frequency standing electric wave within said drift space along said dynode to accelerate said electrons; said waveguide section having a gap for allowing said standing electric wave to interact with said electrons, said gap being smaller than the distance of the furthest end of each of said trajectories from said semicircular dynode as measured in the direction perpendicular to the axis of said waveguide section, and means for applying a static magnetic field within said drift space to deflect the accelerated electrons; whereby said standing wave comprising; means providing a modulated laser light beam; eans further causes said accelerated deflected electrons to photomultiplier means for demodulating said laser light including; an envelope, a photocathode for emitting electrons;

bunch. 

1. A multiplier phototube comprising: an envelope, a photocathode for emitting electrons, an anode for collecting said electrons, at least one dynode to provide secondary emission, said photocathode, dynode and anode being contained within said envelope and defining a drift space through which said electrons may move along chainedsemicircular trajectories, said dynode being disposed within substantially the same plane as said photocathode, a waveguide section disposed substantially parallel to said dynode and having one of its ends shorted for providing a high frequency standing electric wave within said drift space along said dynode to accelerate said electrons, said waveguide section having a gap for allowing said standing electric wave to interact with said electrons, said gap being smaller than the distance of the furthest end of each of said seimicircular trajectories from said dynode as measured in the direction perpendicular to the axis of said waveguide section, and means for applying a static magnetic field within said drift space to deflect the accelerated electrons, whereby said standing wave means further causes said accelerated deflected electrons to bunch.
 2. The phototube of claim 1, in which said waveguide section comprises, a ridge waveguide for forming said gap.
 3. The phototube of claim 1 including, DC-biasing means coupled to said dynode.
 4. The phototube of claim 3 in which said DC-biasing means provides a voltage V0, and the electrons follow a semicircular trajectory, the magnetic field B, direct-current voltage V0, and space 1 between the photocathode and z-axis approximately satisfying the following conditions: the radius of the trajectory being equal to one-fourth of the guide wavelength lambda g; the transit time Tau o of the electron covering the distance 1 equal to being equal to the oscillation period Tau of the high frequency electric field.
 5. A system for demodulating a modulated laser light beam comprising; means providing a modulated laser light beam; photomultiplier means for demodulating said laser light including; an envelope, a photocathode for emitting electrons; means directing the light beam to said photocathode; and anode for collecting said electrons; at least one dynode to provide secondary emission; said photocathode, dynode and anode being contained within said envelope and defining a drift space through which said electrons may move along chained-semicircular trajectories; said dynode being disposed within substantially the same plane as said photocathode; a waveguide section disposed substantially parallel to said dynode and having one of its ends shorted for providing a high frequency standing electric wave within said drift space along said dynode to accelerate said electrons; said waveguide section having a gap for allowing said standing electric wave to interact with said electrons, said gap being smaller than the distance of the furthest end of each of said trajectories from said semicircular dynode as measured in the direction perpendicular to the axis of said waveguide section, and means for applying a static magnetic field within said drift space to deflect the accelerated electrons; whereby said standing wave means further causes said accelerated deflected electrons to bunch. 